One of the key requirements to achieve steady-state power production in a fusion reactor based on the so-called tokamak configuration is to generate non-inductive toroidal plasma current in an efficient way. A recent paper published in Physical Review Letters entitled “Observation of Efficient Lower Hybrid Current Drive at High Density in Diverted Plasmas on the Alcator C-Mod Tokamak” reports on the use of lower hybrid current drive (LHCD) as an efficient mean to obtain non-inductive plasma current at high plasma densities in a diverted tokamak configuration such as Alcator C-Mod.

As explained in the paper, “one of the biggest challenges since the beginning of the LHCD experiments in the 1970s has been to understand and overcome the so-called LHCD density limit-an anomalous loss of efficiency observed at a density below the classical wave accessibility limit”. The paper shows for the first time evidence of efficient generation of non-inductive current above the LH density limit ne = 1 x 1020m-3 in Alcator C-Mod. This has been achieved by increasing the toroidal plasma current up to 1.4MA and keeping ne/nG < 0.2. Furthermore, the paper recognizes the dependence of the scrape-off layer (SOL) parameters on the Greenwald density fraction as a relevant player to obtain efficient LHCD above the LH density limit.

For further information the paper can be downloaded at the PRL webpage here.